1. Field of the Invention
The present invention relates to an imaging apparatus, a radiation imaging apparatus and a radiation imaging system, and more specifically, to an imaging apparatus used in a radiation imaging apparatus and radiation imaging system suitably used for still image radiographing such as plain radiographing and moving image radiographing such as fluoroscopy in medical diagnosis and an imaging apparatus used in a radiation imaging system. In the present invention, radiation includes not only alpha rays, beta rays and gamma rays being beams produced by particles (including photons) emitted by radioactive decay, but beams whose energy are nearly equal to or greater than the above, such as, for example, X rays, corpuscular rays and cosmic rays.
2. Description of the Related Art
In recent years, a radiation imaging apparatus using a flat panel detector (hereinafter, referred to as FPD) formed by a thin film semiconductor material over an insulating substrate has been practically applied as an imaging apparatus used for medical image diagnosis and non-destructive inspection using X rays. The radiation imaging apparatus using the FPD enables a digital radiographing in which radiation such X rays transmitted through an object such as a patient is converted into an analog electric signal by the FPD and the analog electric signal is then converted into a digital image signal. The FPD is roughly classified into two types: direct conversion and indirect conversion. The direct conversion type of a radiation imaging apparatus has an FPD including a two-dimensional area sensor in which a plurality of pixels are two-dimensionally arranged which include conversion elements using semiconductors such as a-Se capable of directly converting radiation into electric charge. The indirect conversion type of a radiation imaging apparatus has an FPD including a two-dimensional area sensor in which a plurality of pixels are two-dimensionally arranged which include a wavelength converter such as phosphor capable of converting radiation into light and a photoelectric conversion element using semiconductors such as a-Si capable of converting light into electric charge. The indirect conversion type of a radiation imaging apparatus is disclosed in U.S. Pat. No. 7,022,997, for example. The radiation imaging apparatus with the FPD is capable of replacing radiation image with digital information, enabling image information to be momentarily transmitted over distance. The radiation imaging apparatus with the FPD is used as a digital imaging apparatus for still image radiographing such as plain radiographing and moving image radiographing such as fluoroscopy in medical image diagnosis, for example.
One method is based on pixel addition. The following describes pixel addition. In general, the two-dimensional area sensor sequentially reads signals from pixels on a row and/or a column basis. However, the more the number of pixels in the two-dimensional area sensor, the longer the time (or one frame time) required to sequentially read out signals from pixels for one frame on a row basis and/or on a column basis. For this reason, a method referred to as “pixel addition” is used as described in Japanese Patent Application Laid-Open No. H07-322141 in which signals are simultaneously read from a plurality of pixels on a plural row basis and/or on a plural column basis to accelerate a scanning speed to shorten one frame time, improving a frame rate. The pixel addition method enables the frame rate to be improved and sensitivity to be improved because the method treats a signal from a plurality of pixels as one synthesized pixel.
Another method is based on trimming. The following describes trimming. Trimming is a method of selectively reading out signals from pixels in a predetermined area. Only signals from arbitrary pixels are sequentially read out on a row basis and/or on a column basis, and signals from other pixels are not read out or signals from other pixels are simultaneously read out. U.S. Pat. No. 6,690,493 discloses the trimming.
However, the pixel addition is lower in resolution than the method in which signals are sequential read out. For this reason, it may be difficult for the pixel addition to adapt to a part such as a heart that moves quickly and has minute vessels.
In the trimming, signals from the pixels outside a predetermined area are not read out or those are simultaneously read out, so that image signals cannot be formed from signals from the pixels outside the predetermined area. Accordingly, when the trimming is used for monitoring when a catheter is inserted or surgical operation is conducted in fluoroscopy, images outside the predetermined area cannot be formed, so that it may be difficult to follow an object moved outside the predetermined area.